In a simple conventional capacitive sensor design, the common electrode, which is mobile, forms part of an armature resiliently held between the two fixed electrodes. In this case, the capacitive sensor may be capable of performing a measurement along one direction of movement of the moving electrode. The moving electrode can move some distance in the direction of one or other of the fixed electrodes under the action of a force.
With this type of sensor with a single measurement axis, the common electrode is idle at an approximately equal distance from the two fixed electrodes, which defines equal capacitive values for the two capacitors. However, when the common electrode moves under the action for example of a force, the capacitive value of each capacitor varies inversely. The electronic interface circuit connected to the capacitive sensor thus enables an analogue output signal to be supplied. This analogue output signal takes the form of a voltage dependent on the capacitance variation of the two capacitors.
This type of electronic interface circuit for a capacitive sensor is disclosed, for example, in the article by Messrs H. Leuthold and F. Rudolph, which appeared in the journal entitled “Sensors and actuators” A21-23 (1990), pages 278 to 281.
The capacitive sensor may be an accelerometer for performing an acceleration measurement in conjunction with an electronic interface circuit. It may be a single axis accelerometer like the aforementioned capacitive sensor, or a multi-axis or tri-axis accelerometer for performing a measurement in three directions X, Y and Z. A tri-axis MEMS accelerometer of this type may include a single mass, i.e. a common inertial mass for the three pairs of differential capacitors, or three masses for the three pairs of capacitors. In the first case, a single common electrode and six fixed electrodes are provided, whereas for the second case, one common electrode with two fixed electrodes are provided for each pair of capacitors.
For a conventional electronic interface circuit for a capacitive sensor, such as a single or tri-axis MEMS accelerometer, the output voltage ideally varies in a linear manner in relation to the movement of the common moving electrode. However, since the electronic circuit is generally integrated in a semiconductor substrate, account must be taken of stray capacitances at the input, which are added to the capacitances of the sensor capacitors. These stray capacitances are practically independent of the movement of the common electrode, which creates non-linearities. Thus, the electronic circuit output voltage does not vary linearly relative to the movement of the common moving electrode. These stray capacitances also have the effect of reducing the sensitivity or gain of the electronic circuit.
The MEMS sensor serving as accelerometer is also integrated in a semiconductor substrate, such as a silicon substrate. This also leads to a problem of non-linearity linked to the potential of the substrate during operation of the sensor. The substrate potential is difficult to control across the entire structure of said sensor, since the substrate is never totally conductive. The moving electrode of the sensor can also be in a shifted position relative to the fixed electrodes in the idle mode, which can create a measuring error without calibration. Because of these non-linearities, the measured electrostatic force is not zero in the sensor and electronic circuit idle mode. Because of the influence of the substrate potential on the electrostatic force, this leads to a variation in the measured real force, which is applied across the common moving electrode, which is a drawback.
Generally, to carry out a force, acceleration or pressure measurement using the electronic circuit, the fixed electrodes of two capacitors or pairs of capacitors are biased or excited cyclically by voltages of opposite polarity relative to an inoperative reference voltage. By biasing or polarizing the two fixed electrodes at different voltage levels, the charge difference across the moving electrode can be measured and converted into at least one electronic circuit output voltage. When the output voltage or voltages are stabilised at their final value, the total charge across the moving electrode becomes zero. These output voltages can be supplied sampled to a processing circuit capable of providing acceleration, force, pressure or also angular speed data depending upon the structure of the sensor.
It is to be noted that conventionally with an integrated electronic interface circuit for a capacitive sensor, the measurement of a force, acceleration or pressure is dependent upon the aforementioned non-linearities and any voltage offset linked to unmatched electronic components. A solution for overcoming this problem has already been proposed in EP Patent Application No. 1 835 263.
In EP Patent Application No 1 835 263, the electronic circuit performs a measurement of a physical parameter, such as an acceleration, by means of a capacitive sensor, which only includes one pair of capacitors operating in differential mode. The common electrode is connected to a conventional charge transfer amplifier, whose output is connected to a first integrator which supplies a first analogue output voltage in a first series of measuring phases, and to a second integrator which supplies a second analogue output voltage in a second successive series of measuring phases. This electronic circuit is thus formed of a double symmetrical structure with the two integrators and also two excitation units for the fixed electrodes operating alternately in total symmetry.
Thus, in the first series of phases, the fixed electrodes are both biased by the first output voltage and biased by the high and low voltage levels of a supply voltage source. In the second series of phases, the fixed electrodes are both biased by the second output voltage and biased inversely to the first series of phases by the low and high voltage levels of the supply voltage source. Because of this, a voltage offset due to technology or to the variation in supply voltage can be minimised or eliminated using the two analogue integrator output voltages. Moreover, the substrate potential is no longer of any great importance given that the electronic circuit is designed with an identical double structure operating in total symmetry.
However, one drawback of this type of electronic circuit of EP Patent Application No 1 835 263 is that it supplies output signals, such as output voltages, in analogue form. This requires the use of two integrators. In these conditions, it is not possible to considerably reduce the size of the integrated components and the electric power consumption of the electronic circuit if the circuit is intended to be integrated in a silicon substrate using CMOS technology of 0.18 μm or less. Moreover, the electronic circuit is only arranged to be connected to one pair of capacitors of a capacitive sensor with a single measurement axis.
WO Patent Application No. 2004/113930, which discloses an electronic circuit connected to a single axis or multi-axis capacitive sensor for measuring an acceleration, can be cited in this regard. In relation to the aforementioned electronic circuit, a specific logic unit for each measurement axis, which processes digital measuring signals, is provided after the charge transfer amplifier, which is connected to the common moving electrode. Each logic unit supplies at output a binary measuring signal representative of a measuring voltage level dependent on the movement of the moving electrode relative to the fixed electrodes for each axis in succession. The binary measuring signal is supplied for each axis in succession to a digital-analogue converter. In one phase of each measuring cycle for a selected axis, this converter supplies a measuring voltage to the fixed electrodes alternately with a phase of polarising the fixed electrodes at a high voltage and a low voltage of a supply voltage source. The binary signal obtained at the output of each logic unit is incremented or decremented by one unit at each series of measuring phases, until the total charge across the moving electrode becomes zero. Owing to the use of the digital signal processing logic unit at the amplifier output, the size of the electronic components can be reduced and consequently also the electric power consumption of the electronic circuit output stages. However, there is nothing provided for removing the aforementioned non-linearities and voltage offsets due to technology or to the variation in supply voltage, which is a drawback. Moreover, the time for precisely stabilising the digital output signal for each measurement axis is relatively long, which is another drawback.
WO Patent Application No. 2008/107737, which discloses an electronic interface circuit for a measuring sensor and a method of activating the electronic circuit, can also be cited. The measuring sensor is formed of two differential connected capacitors for measuring acceleration. An analogue input signal is stored for the measurement after a charge transfer amplifier in one phase of a measuring cycle after the fixed electrodes of the capacitor have been polarised. The analogue signal is then converted into a digital signal stored in a logic unit of the electronic circuit. The digital signal is subsequently converted by a digital-analogue converter into an analogue return signal in the form of a voltage, which is applied to all the sensor electrodes in a successive phase of each measuring cycle. In a measuring cycle, the fixed electrodes are polarized a first time by a first polarization and a second time by a second polarisation that is inverse to the first polarisation. This enables leakage currents to be removed from the electronic circuit. However, a large number of steps of the method are necessary to obtain a physical parameter measuring signal at output, which is a drawback.